Design aspects and optimisation of an axial field permanent magnet machine with an ironless stator

[摘要] ENGLISH ABSTRACT:The advent of new high energy product permanent magnet materials has opened great opportunitiesfor novel electrical machine topologies with advantageous features such as high efficiencyand high power/weight ratio. Amongst others, axial field permanent magnet (AFPM)machines with ironless stators are increasingly being used in power generation applications.Because of the absence of the core losses, a generator with this type of design can operate at asubstantially high efficiency. Besides, the high compactness and disc-shaped profile make thistype of machine particularly suitable for compact integrated power generation systems. Dueto construction problems, the generator application of this type of machine has been limitedto quite a low power range. There is a need to investigate the performance capability of thistype of AFPM machine in the upper medium power level.The focus of this thesis is on the design optimisation of the air-cooled AFPM generator withan ironless stator. A design approach that directly incorporates the finite element field solutionin a multi-dimensional optimisation procedure is developed and applied to the designoptimisation of a 300 kW (at unity power factor) AFPM generator. To enable an overalldesign optimisation of the machine, different design aspects, such as the cooling capacity, themechanical strength and eddy loss, are also studied in this research.To enable the free movement of the rotor mesh with respect to the stator mesh, the air-gapelement originally proposed by Razek et. al. is derived for Cartesian coordinate systems. Forminimising the large computation overhead associated with this macro element, a number ofexisting time-saving schemes have been utilised together with the derived Cartesian air-gapelement. The developed finite element time-step model is applied to calculating the steadystateperformance of the AFPM machine.Since the flux distribution in an AFPM machine is three dimensional by nature, calculatingthe eddy current loss by merely using a simple analytical method may be subject to a significanterror. To overcome this problem, the two dimensional finite element field modellingis introduced to perform accurate field analysis. To exploit the full advantages of the twodimensionalfinite element modelling, a multi-layer approach is proposed, which takes into account the variation of the air-gap flux density in the conductors with regard to their relativepositions in the air-gap. To account for the radial variation of the field, a multi-slice finiteelement modelling scheme is devised.The thermal analysis is an important aspect of the design optimisation of AFPM machines.From a design point of view, it is preferable to have a simple but effective method for coolinganalysis and design, which can easily be adapted to a wide range of AFPM machines. Inthis thesis a thermofluid model of the AFPM machine is developed. The fluid flow model isneeded for calculating the air flow rate, which is then used to find the convective heat transfercoefficients. These are important parameters in the subsequent thermal calculations.Experimental investigations have been carried out to verify each of the above-mentionedmodels/methods. With these models implemented, the design optimisation of an air-cooledironless stator 300 kW (at unity power factor) AFPM generator is carried out. The performancemeasurements done on the fabricated prototype are compared in this thesis withpredicted results. The study shows that the proposed design approach can be applied withsuccess to optimise the design of the AFPM machine. The advantages of high power density,high efficiency, no cogging torque and good voltage regulation make this type of AFPM machinevery suitable for power generator applications. The optimum steady-state performanceof the AFPM machine shows that this machine with an ironless stator is an excellent candidatefor high speed power generator applications, even in the upper medium power level.The good cooling capacity of this type of machine holds the promise of its being a self-cooledgenerator at high power ratings.

[发布机构] Stellenbosch University